1999 chevy silverado 5.3 eng starts then immediatly quits

If you have just got a new or different key, it might not be correctly programmed to your truck. That's what mine did when I got a new key.

Theres a theft security system built into your PCM Try to turn the key to on (not start) and leave it on for 10 minutes, then key off, for 30 seconds, then try and start

check the ignition moduel in the distrbituer

First... Check the fuel supply... Both quality (no water or scum/gums etc.) AND the delivery pressure.

Your checks should include the electrical side of the fuel delivery system... esp. the fuel pump fuse and it's relay. If the fuel pump 'relay' is blown... ok; replace it.

If the fuel pump 'fuse' is blown (very rare!) ... STOP! Seek a qualified auto electrician to find out why! Remember a fuse is designed for many reasons...including to stop sparks! Fuel and sparks? Don't bother! Get a qualified auto electrician!

Note = It is normal proceedure to disable (disconnect/remove) this fuse and relay; then crank and run the motor until engine failure, or black out...Specifically to de pressurize the fuel lines for a fuel filter replacement!

Second.... Check the air supply. Is your air filter clean? Is the route from your filter to your intake clear of obstructions? => Anti pollution vaccuum tubes, incorrectly connected, can also cause black outs!

If it was an ignition (electrical) problem the motor just would not start in the first place, or sometimes it would and sometimes not.

If it was incorrect timing you would have backfiring.

I believe the problem is likely to be with the fuel supply. (Including air, as this too is a part of an engines fuel!)

Hope this helps! :-)